The present invention relates generally to hydroforming die assemblies, and more particularly to a hydroforming die assembly which prevents the metallic tubular blank to be hydroformed from being pinched during closure of the die assembly.
Hydroforming methods are commonly known as a means for shaping a tubular metal blank into a tubular component having a predetermined desired configuration. In particular, a typical hydroforming operation involves the placement of a tubular metal blank into a hydroforming die cavity and providing high pressure fluid to the interior of the blank to cause the blank to expand outwardly into conformity with the surfaces defining the die cavity. More particularly, the opposite longitudinal ends of the tubular metal blank are sealed, and high pressure water is provided through a hydroforming port or ram sealing one of the tubular ends. The fluid provided within the tube is pressurized by a conventional intensifier.
Typically, the die assembly includes a lower die half and an upper die half. The upper die half moves downwardly to cooperate with the lower die half to form the sealed die cavity therebetween. The tubular metal blank is placed in the lower die half before the upper die half is lowered to seal the tubular blank within the cavity.
For many applications, the tubular blank, which typically has a circular cross-section, is hydroformed into a tubular part or component having a boxed or rectangular cross-section as defined by the die cavity. Because the circumference of the tubular blank is significantly less than the circumference or cross-sectional perimeter of the surfaces defining the die cavity, it is often desirable to slightly crush or deform the tubular blank within the die cavity as the upper die half is lowered to seal the die cavity. The desirability of slightly deforming the tubular blank within the die cavity prior to pressurizing the tube for expansion stems, in part, from the need to conform the cross-sectional perimeter of the tubular blank more closely to the cross-sectional perimeter or circumference of the surfaces defining the die cavity to alleviate some of the need to expand or stretch the metal material of the tubular blank during the pressurizing phase of the hydroforming operation. In addition, providing a tubular blank with a cross-sectional perimeter which more closely conforms to that of the die cavity (which can be viewed as providing some "slack" in the metal material for facilitating expansion thereof into conformity with the die cavity) facilitates the ability for expansion of the tubular blank into the "hard" corners of the die cavity.
A problem encountered during the deformation of the tubular blank upon closure of the die cavity is the possibility of the deformed tubular blank to become pinched between the upper and lower die halves as the die cavity is sealed. One solution to this potential problem is discussed in U.S. Pat. No. 4,829,803. This patent discusses an arrangement wherein the tubular blank must be pressurized sufficiently prior to lowering the upper die half, and the exterior surface of the blank must be smoothed sufficiently, such that the internal pressure within the tubular blank prior to the upper die half being closed is at least sufficient to overcome the frictional forces exerting on the blank by the die sections on closing of the die sections. This construction places a degree of criticality on the internal pressure within the tubular blank and the smoothness of various friction surfaces. In addition, because the die assembly deforms the tube before the die cavity is sealed, the pinching problem remains a possibility.
An alternate proposal in U.S. Pat. No. 5,339,667 likewise requires deformation of the tubular blank prior to sealing of the die cavity. This, again, creates the possibility of pinching the tube upon closure of the die cavity. In addition, this patent provides a die cavity with very specific contours to take into account the possibility of pinching the tubular blank. Thus, only limited shapes of tubular components can be formed by this process.
U.S. Pat. No. 5,239,852 provides yet another proposal to solving this problem. However, in this arrangement two die structures must come together with a very high degree of precision to make certain that each of the side walls of the die cavity come into close proximity with sealing surfaces of the opposing die structure. In addition, this construction provides a severely acute angle at the transition between the ledge and heel of the die structures. This corner, formed at such an acute angle, provides a relatively weak portion of the die structure which may be subject to chipping or cracking after prolonged use.
It is object of the invention to overcome the difficulties in the prior art noted above. The present invention accomplishes this by providing at least three separate die structures cooperable to define a die cavity into which a metallic tubular blank can be disposed. The first die structure is moveable to seal the die cavity, and after the die cavity is sealed, the first and second die structures are moveable to reduce the cross-sectional area of the die cavity and thereby deform the metallic tubular blank within the die cavity.
Also in accordance with the present invention, two moveable die structures and a single fixed die structure are provided to define the die cavity. Relative movement between the first and second movable structures seals the cavity. After the cavity is sealed, movement of the first die structure relative to the fixed die structure reduces the cross-sectional area of the die cavity to deform the metal tube in the die cavity.
It is a further object of the present invention to provide a method of hydroforming a metallic tube. The method comprises placing the metallic tube in a hydroforming die assembly having three separate die structures, the three die structures being cooperable to define a die cavity; moving a first one of the die structures to seal the die cavity; then moving the first one of the die structures and a second one of the die structures to reduce the cross-sectional area of the die cavity; and deforming the metallic tube as a result of reducing the cross-sectional of the die cavity.
A further object of the invention is to provide a hydroforming die assembly comprising a lower die assembly defining a lower die cavity portion into which a metallic tube can be placed, the lower die assembly providing side walls defining opposite sides of the lower die cavity portion, and a lower wall defining a lower surface of the lower die cavity; an upper movable die structure having sealing surfaces which are movable to engage the lower die assembly on opposite sides of the lower die cavity portion to seal the lower die cavity portion and thereby provide a sealed die cavity; the lower die assembly and the upper die structure being cooperable to reduce a size of the sealed die cavity to deform the metallic tube after the die cavity is sealed.
Other objects and advantages of the present invention will be realized in accordance with the following detailed description, appended drawings and claims.